A non-return valve allows a medium to flow in only one direction. A non-return valve is fitted to ensure that a medium flows through a pipe in the right direction, where pressure conditions may otherwise cause reversed flow.

A non-return valve can be fitted to ensure that a medium flows through a pipe in the right direction, where pressure conditions may otherwise cause reversed flow. A non-return valve allows a medium to flow in only one direction. The flow through the non-return valve causes a relatively large pressure drop, which has to be taken into account when designing the system.

There are different types of non-return valves, such as spring-loaded, swing type, and clapper type valves. Non-return valves are e.g. used with mixing loops in heating and cooling systems to ensure proper operation, and with domestic water systems to prevent backflow.